1. Technical Field of the Invention
The present invention relates to the generation of a control voltage for a cascode transistor in an open drain transmitter configuration and, more specifically, to the generation of such a control voltage in a power-off condition to address reliability issues.
2. Description of Related Art
Reference is made to FIG. 1 which shows a circuit diagram of a prior art open drain transmitter configuration. A first pad 10 of a transmitter integrated circuit 12 is connected to a termination voltage Vterm through an external termination resistor 14. The integrated circuit 12 includes a cascode MOS transistor 16 (for example, of the NMOS-type) having its drain connected to the first pad 10 in an open-drain configuration. The source of the cascode MOS transistor 16 is connected to a second pad 18 (for example, a ground reference voltage pad) of the integrated circuit 12 through a current switch 20. The gate of the cascode MOS transistor 16 receives a reference (bias) voltage Vref which is derived by a bias generator 22 from an integrated circuit supply voltage Vsup received at a third pad 24. The reference (bias) voltage Vref is equal to or less than the supply voltage Vsup.
A concern exists with respect to this prior art circuit configuration when the transmitter integrated circuit 12 is not powered by the supply voltage Vsup at the third pad 24 but the termination voltage Vterm remains present. Such might occur, for example, in a situation where power is shut off to transmitter integrated circuit 12 but a receiver circuit associated with the termination voltage Vterm continues to be powered. In such a case, the reference (bias) voltage Vref at pad 24 falls to zero and the voltage gate-to-drain and voltage drain-to-source for the cascode MOS transistor 16 will equal the termination voltage Vterm. If the termination voltage Vterm exceeds an absolute maximum rating (AMR) of the cascode MOS transistor 16, then issues concerning transmitter integrated circuit 12 reliability can arise.
In technologies where oxide thickness is usually large enough, the oxide reliability of a cascode MOS transistor in an open drain transmitter configuration is not of major concern. The reason for this is that the fabricated transistor typically can tolerate voltages associated with the termination voltage Vterm. However, in the technologies where the oxide is thinner, concerns with oxide reliability arise because open drain MOS transistors in an off condition typically cannot take voltages across their terminals in excess of the integrated circuit supply voltage Vsup. If the termination voltage Vterm exceeds the integrated circuit supply voltage Vsup, there is a risk that the open drain MOS transistor in an off condition will be damaged.
A need exists in the art to address the foregoing problem.